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This page has the questions and answers for issues related to objects in the
night sky.
- 4/10/00 - Sophia Hu - "We are teaching an astronomy unit this
week, and would like to know if the lunar phases are the same in the
southern hemisphere as they are here"
Ans: [djt] Of course they are. Lunar phases are caused by the
geometry of the Sun, Earth, and Moon, and that geometry is independent
of whether you are viewing from the northern or southern hemisphere.
The easiest way to demonstrate this independence is to draw the
situation. To have a full Moon, the Sun and Moon must be on opposite
sides of the Earth. The Moon is still opposite the Sun regardless of
whether you are in the northern or southern hemisphere.
Now, having said that, there is a tiny parallactic effect that does
depend on where the observer is located, but it's just as much an
east-to-west effect as it is a north-to-south effect. For example, to
have a precisely full Moon, the observer needs to be exactly on the
line connecting the centers of the Sun and the Moon, and only one spot
on the surface of the Earth will interest that line (while also being
able to view the Moon; there is, of course, another intersection point
on the other side of the Earth, but you won't be able to see the Moon
from that point). Move away from that point, and you won't see a
precisely full Moon, but the effect is not noticeable to the casual
observer.
When the calendar says First Quarter, the Sun illuminates Mare Crisium
while Mare Imbrium is in shadow. That situation is the same for all
observers on Earth, northern and southern hemisphere.
However, the position of the crescent will look different as a function
of latitude on Earth.
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